The present invention relates to plasma processing apparatuses and, more particularly, to a plasma processing apparatus which is capable of performing a high performance plasma process and has a high electric-power-efficiency and a long maintenance period.
In recent years, in order to realize semiconductors and liquid crystal displays having a high-performance and high-throughput, a plasma process has become indispensable for manufacturing these products. Although there are various methods for plasma excitation, a parallel plate type RF plasma excitation apparatus or a inductive coupling type plasma apparatus has been used to manufacture semiconductors or liquid crystal displays. These plasma apparatuses have several essential problems in that a large damage is given to a device and a high-performance process at a high-speed cannot be achieved. Accordingly, it has become difficult to satisfy demands of semiconductors and liquid crystal displays to have a high-performance and high-throughput
Accordingly, a microwave plasma apparatus has recently attracting attention, which can excite high-density plasma by a microwave electric field without using a direct current magnetic field. As such kind of micro plasma apparatus, an apparatus (Japanese Laid-Open Patent Application No. 9-63793) is known, which excites plasma by ionizing a gas in a vacuum chamber by a microwave electric field generated by a microwave emitted to the vacuum chamber from a flat antenna (slot antenna) having many slots that are arranged to generate a uniform microwave. Additionally, there is also known an apparatus (WO98/33362), which excites plasma by introducing a microwave, which is emitted by a slot antenna provided outside a vacuum chamber, into the vacuum chamber by being passed through a dielectric material separation wall and a dielectric material shower plate. Since the microwave plasma excited by those methods has a high-density and a low electron temperature, a process having no damage at a high speed can be performed. Additionally, since uniform plasma can be excited even on a large area substrate, it can be easily dealt with an increase in the size of a semiconductor substrate or a liquid crystal display.
However, these conventional microwave plasma apparatuses have a problem in that a substance, which is produced by dissociation and combination of a process gas due to the plasma, adheres onto a surface of the dielectric material separation wall or the shower plate. If a film having a low resistivity is deposited on the surface, the microwave is reflected, and if a film having a high resistivity, the microwave is absorbed. Accordingly, the plasma excitation power is decreased due to adhesion of the substance onto the surface of the dielectric material separation wall or the dielectric material shower plate, which reduces the plasma density and deteriorates stability of the plasma. In the worst case, it becomes a situation in which the plasma cannot be exited. In order to eliminate such a problem, it is necessary to frequently perform a chamber cleaning and maintenance so as to remove the adhered film, which significantly decreases the throughput.
In the reactive ion etching which is indispensable for producing semiconductors or liquid crystal displays, anisotropic etching is achieved by irradiating ions in the plasma onto a substrate surface by accelerating up to 100 eV by an electric filed in a sheath formed between the substrate and the plasma. In order to generate a direct current voltage (self bias voltage) for accelerating the ions to a desired energy in the sheath near the substrate, an RF wave ranging from about several hundreds KHz to about several tens MHz is applied to the substrate. Since the plasma can be regarded as a conductive material, the RF voltage applied to the substrate is divided into that the sheath near the substrate and the sheath near the grounded part. That is, if the RF wave is applied to the substrate, the RF voltage is applied not only to the sheath near the substrate but also to the sheath near the grounded part, and, thereby, the DC voltage of the sheath near the grounded part is increased and a plasma potential is increased. If the plasma potential becomes greater than 15 to 30 V, contamination occurs due to sputtering of the surface of the grounded part due to bombardment of the accelerated ions.
A ratio of the RF voltages applied to the sheath near the substrate and the sheath near the grounded part is determined by impedances of these sheathes. If the impedance of the sheath near the grounded part is much smaller than the impedance of the sheath near the substrate, a most part of the RF voltage applied to the substrate is applied to the sheath near the substrate. That is, if the area of the grounded part to which the plasma contacts is sufficiently larger than the area of the substrate electrode (normally, more than four times), the plasma potential is not increased when a RF wave is applied to the substrate. Thus, a problem associated with the contamination due to the sputtering can be avoided. Additionally, a large DC voltage can be efficiently generated in the sheath near the substrate.
However, in the conventional microwave plasma apparatus, since the opposing surface of the substrate is covered by a dielectric material in its entirety, the area of the grounded part to which the plasma contacts cannot be large. Normally, an area of the grounded part to which the plasma contacts is less than about three times the area of the substrate electrode. Accordingly, it is difficult to apply to an reactive ion etching such as a process in which a high energy ions must be bombarded to a substrate surface.
In a process for forming a thin film containing a metal such as metal thin film , feroelectric film, and high dielectric thin film by CVD (chemical vapor deposition) method, and an organometallic gas which is a compound of metal atoms and organic molecules is used. If the bonds between the metal atoms and the organic molecules is selectively cut off, a thin film having a good characteristic which causes no impurity contamination will be formed. However, if the organic molecules are decomposed, a large amount of carbon impurity atoms are mixed into the film, which deteriorates the characteristic of the thin film. Additionally, in the etching process, if the dissociation of the process gas progresses in excess, the selectivity between the film to be etched and a resist mask or the underlying material is deteriorated, and it becomes difficult to etch a fine pattern having a large aspect ratio. In the conventional microwave plasma processing apparatus, the process gas is directly introduced into an area close to a position at which the microwave is incident and having a high plasma density and a relatively high electron temperature. Thereby, the dissociation of the process gas progresses in excess, and a good result cannot be obtained in formation of a thin film using an organometallic gas or fine pattern etching.
When a microwave is incident on plasma, the microwave propagates in the plasma if the electron density if smaller than the cutoff density nc represented by the following equation.
nc=xcex50xcfx892m0/e2
where xcex50 is a permittivity of dielectric ratio of vacuum; xcfx89 is microwave angular frequency, m0 is a mass of an electron, and e is a charge of an electron. On the other hand, if the electron density is higher than the cutoff density, the microwave is reflected in the vicinity of a plasma surface. At this time, the microwave penetrates into the plasma by a penetration length (normally, several millimeters to ten millimeters), and gives energy to electrons in the plasma so that the plasma is maintained. In to the microwave plasma excitation, if the electron density is lower than the cutoff density, uniform and stable plasma cannot be excited due to dispersion of the microwave in the chamber. In order to excite uniform and stable plasma, it is indispensable to reflect a large part of the microwave by exciting plasma having an electron density sufficiently higher than the cutoff density in the vicinity of the surface on which the microwave is incident. In order to excite a stable plasma having a high electron density, an inert gas such as amyR is preferably used as the plasma excitation gas. If a gas other than a monatomic molecule gas is added, it tends to deteriorate the stability of the plasma due to the electron density being decreased since the energy of the microwave is used for dissociation of the gas molecules. In the conventional microwave plasma apparatus, since only a small amount (several percent) of gas other than the inert gas can be added, there is a problem in that process window is narrow and it cannot deal with a high speed process.
When the electron density in the vicinity of the plasma surface is higher than the cutoff density, a large part of the microwave incident on the plasma is reflected in the vicinity of the surface. The reflected wave is received by the slot antenna, and, thereafter, emitted form the slot antenna by being reflected by a matching unit connected between the slot antenna and the microwave power source. The microwave gradually provides its energy to the plasma while repeatedly reflected between the plasma surface and the matching unit. That is, the microwave is in a resonant state in a part between the plasma surface and the matching unit. Accordingly, a high energy density microwave is present in this part, and a large loss is caused due to a small conductive loss of a metal wall of the waveguide or a small dielectric loss of the dielectric material. In the conventional microwave plasma apparatus, these losses are large, and, thereby, the plasma excitation power efficiency was low. Additionally, if a large power microwave is supplied so as to obtain a high-density plasma, an arc discharge is generated in a slot part formed on the surface of the slot antenna. Thereby, there is a problem in that the antenna is broken or a discharge occurs in a gas passage between the dielectric material separation wall and the dielectric material shower plate.
It is a general object of the present invention to provide an improved and useful plasma processing apparatus in which the above-mentioned problems are eliminated.
A more specific object of the present invention is to provide a plasma processing apparatus which can generate a plasma having high stability even if any process gas is used since there is no film deposition due to dissociation and combination of the process gas on a surface of a dielectric material shower plate of a microwave introducing part.
It is another object of the present invention to provide a plasma processing apparatus of which chamber cleaning period or a maintenance period is long.
It is a further object of the present invention to provide a plasma processing apparatus which can deal with a process in which a high-ion-energy must be incident on a substrate surface.
Additionally, it is another object of the present invention to provide a plasma processing apparatus which can perform an excellent film deposition process or etching process due to an appropriate control of dissociation of the process gas, and can achieve a high plasma excitation efficiency.
In order to achieve the above-mentioned objects, a new process gas discharge unit (referred to as a lattice-like shower plate) is provided to a plasma diffusion part (between a dielectric material shower plate and a substrate) of the conventional microwave plasma processing apparatus so that the plasma excitation gas mainly containing an inert gas and the process gas can be discharged from different locations. Additionally, by grounding the metal made lattice-like shower plate, the apparatus can deal with a process in which high-energy ions must be incident on a surface of the substrate which process cannot be performed by a conventional microwave plasma processing apparatus. Further, a thickness of a dielectric material part of a microwave introducing part is optimized so as to maximize an efficiency of plasma excitation, and, at the same time, a thickness of the dielectric material shower plate and a distance between a slot antenna and a dielectric material separation wall are optimized so as to be capable of supplying a microwave having a large power.
The plasma processing apparatus of the present invention has a structure in which the new gas discharging means (lattice-like shower plate) is provided between the dielectric material shower plate of the conventional microwave plasma processing apparatus and a substrate, and a process gas of which dissociation is preferably suppressed is discharged toward the substrate. On the other hand, in order to prevent the process gas from diffusing toward the dielectric material shower plate, the plasma excitation gas mainly containing an inert gas is discharged from the dielectric material shower plate which is located on opposite side of the lattice-like shower plate with respect to the substrate. Since a film is not deposited on a surface of the dielectric material shower plate which is a path of the microwave, a chamber cleaning period or a maintenance period can be increased, and a stable plasma can be always obtained. Additionally, since a state in which a process gas rarely present can be formed near a surface on which a microwave having a high plasma density and a high electron temperature is incident, the dissociation of the process gas is suppressed and a high-performance process can be achieved. At the same time, since stable plasma having a high density sufficiently greater than a cutoff density can be excited near a surface on which the microwave is incident even if a large amount of process gas is discharged from the lattice-like shower plate, a freedom of the process is greatly improved, and a higher-speed process can be achieved.
An area of the grounded part to which the plasma contacts is greatly increased by introducing the grounded metal made lattice-like shower plate into the plasma. When a RF bias is applied to the substrate, a large part of the RF voltage can be applied to a sheath near the substrate, and, thereby, the energy of ions incident on the surface of the substrate can be efficiently increased without increasing a potential of the plasma. Accordingly, the present invention can be effectively applied to a process such as a reactive ion etching in which a high ion-energy must be incident on a surface of a substrate.
Additionally, according to the plasma processing apparatus of the present invention, an efficiency of excitation of the plasma is maximized by optimizing a thickness of the dielectric material part of the microwave introducing part (a thickness of the dielectric material separation wall plus a thickness of the dielectric material shower plate), and, at the same time, a microwave having a large power can be supplied by optimizing a thickness of the dielectric material shower plate and a distance between the slot antenna and the dielectric material separation wall, and, thereby a more stable and high-density plasma can be efficiently produced. The metal-made lattice-like shower plate is constituted by a stainless steel and aluminum having an aluminum oxide film which has an excellent resistance to the plasma of a corrosive gas, and is usable for a long time.
Other objects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings.